The present invention relates to a method and a device for controlling a sensorless electric motor, in particular a machine tool drive. The method and device find applications in technical fields in which machine tools are driven by an electric motor.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Sensorless operation of an electric motor, in other words operation of an electric motor without a rotational speed sensor, is of considerable importance in many applications for driving machine tools. Reasons for this are firstly cost savings and secondly the reliability of the drives, since without speed sensors no failures induced thereby can be caused either.
A sensorless electric motor that is used within a drive, in particular a machine tool drive, is driven in a open-loop control mode of operation in the lower speed range, for example within a speed range of approximately 10% of the rated speed. The reason for this is that in this range the motor model cannot provide an actual value for the speed, which value is required in order to regulate the electric motor. If the electric motor is accelerated or decelerated during sensorless operation with an excessively high change in frequency or an excessively high speed stroke, there is a risk that the electric motor will “buck”. The term “bucking” of an electric motor means in this context that the electric motor is unable to follow the change in frequency or the required speed stroke and as a result has reached a maximum permitted slip value, for which reason the torque of the electric motor collapses abruptly. The consequence of this is that the electric motor no longer accelerates or no longer delivers a torque required for the drive.
If, on the other hand, the electric motor is accelerated or decelerated with a correspondingly low change in frequency or a correspondingly low speed stroke, time available for accelerating and decelerating would not be used.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved to method for controlling an electric motor which is used in particular in a machine tool drive and which enables the electric motor to accelerate or decelerate in a maximally time-saving manner. A further object is to disclose a corresponding device for performing the method.